Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products, particularly gypsum wallboard. Gypsum is a plentiful and generally inexpensive raw material which, through a process of dehydration (or calcination) and rehydration, can be cast, molded or otherwise formed into useful shapes. The base material from which gypsum wallboard and other gypsum products are manufactured is the hemihydrate form of calcium sulfate (CaSO4.½H2O), commonly termed “stucco,” which is produced by heat conversion of the dihydrate form of calcium sulfate (CaSO4.2H2O), from which 1½ water molecules have been removed. After rehydration, the hemihydrate dissolves, gypsum crystals precipitate, and the crystal mass sets and becomes solid, providing a set gypsum material.
In order to make gypsum-containing products, gypsum-containing slurries are generally prepared. The gypsum-containing slurries may contain stucco and water, and other known ingredients and additives, such as, for example, starch, dispersants, accelerators, binders, glass fibers, and paper fibers, etc. Normally, dry starch is added to the dry stucco and is introduced into a board mixer on the production line along with the other dry ingredients.
Starch is known to be a good binder for gypsum crystals within a set gypsum core, and can increase board strength. In addition, starch can function to provide an improved interface bonding between the set gypsum core and a cover sheet in a wallboard product.
Use of a fine ground pregelatinized corn starch (for example, where >99% of particles pass through a 100 mesh screen), in gypsum-containing slurries is known in the art. However, the fine ground starch requires extra energy and costs more to produce and it also produces a fine dust that is difficult to handle during its production and application. Another problem is that the fine starch can require additional process water when mixing with stucco and water to make a desired gypsum containing slurry. For example, one pound of dry fine ground starch can increase water demand in gypsum slurry preparation by about 10 lb/MSF (1,000 square feet).
Coarse starch, that is larger particle size starch, can be used in the gypsum-containing slurry, since it is less dusty and easier to produce and to handle. In addition, larger particle sizes require less water in the slurry. However, due to the larger particle size it is difficult to fully disperse coarse starch in the gypsum slurry using a standard dry basis process equipment, such as during very short-term mixing with water and stucco through a board mixer where contact time is typically less than one second. If a way could be found to effectively disperse larger particle size pregelatinized starch in the gypsum slurry, water usage could be reduced and cost savings realized. Additionally, if the use of a larger particle size pregelatinized starch also increased board strength, this would serve as another useful contribution to the art.
Furthermore, if a way could be found to effectively prepare a suitable pregelatinized starch dispersion before introduction to the board mixer on a standard production line, this would represent a useful contribution to the art.